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SSE transport: connection succeeds then marked 'Failed' after first event, works fine with raw curl

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Valutazione

Difficoltà
4/5
Tempo stimato
1-2 giorni
Idoneità per principianti
53/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
typescript
Ambito
backend-api-design

Direzione di ricerca

Start by tracing the Inspector legacy SSE client path used for the /sse connection, then inspect how it handles the endpoint event and stream lifetime. Reproduce with a Python MCP SDK SSE server or the reported request sequence; done means the Inspector keeps the stream open and proceeds to initialize and list tools. The issue does not name source files or tests, so finding the relevant entry point will require reading around the transport implementation.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Describe the bug

Connecting to a remote MCP server using the legacy SSE (Server Sent Events) [deprecated] transport: the initial GET /sse request succeeds (200, correct event: endpoint frame delivered with a valid session_id), but the Inspector UI then marks the server connection status as "Failed" and never proceeds to list tools/resources. The History panel shows the single GET transport request as 200 SSE, with the response body described as "Long-lived stream — 1 event delivered, closed; body not captured".

Driving the exact same server by hand with raw curl (bypassing Inspector entirely) completes the full handshake correctly: GET /sse stays open indefinitely, and POSTing initialize → notifications/initialized → tools/list → tools/call to the returned /messages/?session_id=... endpoint all return correct JSON-RPC responses on the SSE stream, with no server-side errors. This confirms the target server, TLS, and auth header are all fine — the failure is isolated to Inspector's own legacy SSE client path.

Steps to reproduce

  1. Start Inspector against a remote MCP server using the Python MCP SDK's SSE transport (uvicorn-based, /sse + /messages/?session_id=... pattern): 
    npx @modelcontextprotocol/inspector \
  --transport sse \
  --server-url "https://<your-server>/sse" \
  --header "Authorization: Bearer <redacted>"
  2. Open the printed Inspector UI URL, confirm Transport = SSE, Server URL ends in /sse, Authorization header populated.
  3. Click Connect.
  4. UI shows "Connecting..." then settles on "Failed".
  5. In the Requests panel, the single GET transport request shows 200 SSE with 1 event delivered, then the connection is reported closed.

Expected behavior

Inspector should keep the SSE stream open (the same way a direct curl -N connection to the identical endpoint stays open indefinitely) and proceed to auto-initialize / allow listing tools, since the server is fully spec-compliant and responds correctly when driven manually.

Environment

  • Inspector version: latest via npx @modelcontextprotocol/inspector (package version 2.12.0 as of testing)
  • OS: macOS (darwin)
  • Target server: Python MCP SDK, SSE transport, fronted by a cloud load balancer (TLS terminated there)

Additional context

Manually replayed protocol trace (via curl) against the same server, for reference, showing a fully successful initialize → tools/list → tools/call exchange with no errors — confirming the server side is healthy and the bug is isolated to Inspector's legacy SSE client:

event: endpoint
data: /messages/?session_id=<redacted>

event: message
data: {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05","capabilities":{...},"serverInfo":{"name":"<server-name>","version":"<x.y.z>"}}}

event: message
data: {"jsonrpc":"2.0","id":2,"result":{"tools":[{"name":"tool_a", ...}, {"name":"tool_b", ...}, {"name":"tool_c", ...}]}}

event: message
data: {"jsonrpc":"2.0","id":3,"result":{"content":[{"type":"text","text":"<tool output>"}],"isError":false}}

Happy to provide a full HAR/network export or more detail if useful — the server itself cannot be shared since it's a private internal instance.

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